feat: add home page with search/unassigned tables; center login card

- Add home view as default: search by inv number, unassigned equipment
  table with auditory assignment, search results (assigned) table
- Add inv_number query param to GET /oboruds/ for backend search
- Center login card vertically and horizontally via flexbox

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/oboruds.py

@@ -15,12 +15,14 @@ def create_oborud(item: schemas.OborudCreate, db: Session = Depends(database.get
     return obj
 
 @oboruds.get("/", response_model=list[schemas.OborudRead])
-def list_oboruds(aud_id: Optional[int] = None, sort_by_inv: bool = False, unassigned: bool = False, db: Session = Depends(database.get_db)):
+def list_oboruds(aud_id: Optional[int] = None, sort_by_inv: bool = False, unassigned: bool = False, inv_number: Optional[int] = None, db: Session = Depends(database.get_db)):
     query = db.query(models.Oboruds)
     if unassigned:
         query = query.filter(models.Oboruds.aud_id == None)
     elif aud_id is not None:
         query = query.filter(models.Oboruds.aud_id == aud_id)
+    if inv_number is not None:
+        query = query.filter(models.Oboruds.invNumber == inv_number)
     if sort_by_inv:
         query = query.order_by(models.Oboruds.invNumber.asc())
     return query.all()